To satisfy the demand for high data rate specified by fourth-generation (4G) mobile communications, two key techniques are adopted. One of the two key techniques is universal frequency reuse (UFR) for enhancing spectrum utilization efficiency of a system. The UFR technique however results in severe co-channel interference, which is most significant for cell edge users (CEUs). Therefore, the other technique, namely network multiple-input multiple-output (MIMO), is utilized for enhancing communication quality for CEUs. In two main 4G standard organizations, Long Term Evolution Advanced (LTE-A) and Worldwide Interoperability for Microwave Access (WiMAX), network MIMO is respectively referred to as coordinated multi-point (CoMP) and collaborative MIMO (Co-MIMO). An intention of the network MIMO is to combine multiple antenna signals by coordinating collaboration among multiple neighboring base stations, so as to solve the issue that CEUs are in long-term exposed to unsatisfactory communication quality under the universal frequency reuse technique. Thus, without increasing a bandwidth or a total transmission power, through signal processing of multiple antennas at transmission terminals and a reception terminal, spectrum efficiency of a wireless communication system can be remarkably enhanced to further increase the data rate and to improve communication quality. In a network MIMO system, each CELT has a corresponding cooperative cell set (CCS), and is collaboratively serviced by all base stations within the corresponding CCS.
The US publication No. 2012115469 discloses method for user equipment (UE) indication and measurement for interference coordination. Through the UE, limited radio resources are confirmed and it is not necessary to receive a precise measurement configuration. Further, through the UE, interference statuses and/or additional interference information of the UE may be reported to a corresponding base station to reinforce interference coordination. In addition, measurement results of the UE may also be utilized for scheduling, RLM and mobility management to further optimize radio spectrum performance as well as to improve user experiences.
In an MIMO network, in order to reduce interference between cells and to enhance CEU performance through collaborative transmission between multiple cells, the research interest in MIMO networks and inter-cell interference coordination (ICIC) such as fractional frequency reuse (FFR) in cellular mobile communication systems continues to grow. The study of ICIC strategies on top of network MIMO systems has drawn increasing interest, where cells are divided into sectors for efficiently managing radio resource and mitigating inter-cell interference. However, these results were proposed without proper analysis on a close relationship between cell sectorization and a system capacity. FIG. 1 shows a conventional sectorization method, In FIG. 1, a central coverage area 1 neighboring to a center 3 and an edge coverage area 2 farther from the center 3 are depicted. As the central coverage area 1 is closer to the center 3, a user in the central coverage area 1 is given better wireless signal transmission and reception quality, whereas a user at the edge coverage area 2 farther away from the center 3 receives weaker wireless signals and also suffers from severer inter-cell interference. A directional antenna 4 associated with a sector points to a central position of a border 5, and so a coverage area of each directional antenna 4 forms a triangular shape in a hexagonal cell. Signals transmitted along a direction pointed by each of the directional antennas 4 have the strongest strength, whereas signals transmitted along other angles towards two sides of the directional antenna 4 are gradually weakened. Referring to FIG. 2 showing a curve 6 of antenna gain, when the direction of the directional antenna 4 is a reference angle (0 degree), the coverage area of the directional antenna 4 is 0 to −30 degrees extended to the left and 0 to +30 degrees extended to the right. Note that a user is located closer to the angle of 0 degree (the direction pointed by the directional antenna 4), better signals are obtained. As the location of the user is getting towards the two sides of the directional antenna 4, signals are gradually weakened, thus damaging the signal quality of wireless transmission. A conventional hexagonal cell is sectorized into triangles for 60-degree sectorization. Referring to a particular edge coverage area 2 in FIG. 1, a height of a particular bar 7 in FIG. 2 represents an edge area to a particular corresponding one angle span. In other words, as a deflection angle from the directional antenna 4 gets larger, the area occupied by the corresponding bar 7 in the edge coverage area 2 of the triangle becomes greater. Further, as observed from FIG. 2, the bars 7 are inversely proportional to the antenna strength curve 6. That is, as an included angle of the directional antenna 4 gets larger, a corresponding edge area ratio occupied also becomes larger. Thus, the edge area of a sector is mainly distributed in regions far away from the direction giving the maximum antenna gain. In general, users are uniformly distributed in a cell. For the conventional sectorization method, it follows that most CEUs are located in regions with less transmitted antenna gains, which unfortunately cause that most user signals transmitted are weakened due to their corresponding smaller antenna gains.
Also, referring to FIG. 3, in the conventional cell sectorization, a CCS corresponding to a CEU 8 located at the edge coverage area 2 only includes two neighboring collaborative base stations (one its own home base station and one neighboring base station). Each of the two base stations has a directional antenna exactly pointing towards each other. Thus, a CEU located in the edge coverage area 2 is provided with only one additional neighboring cell for signal connection and data transmission. Further, the CEU also receives a greater interference strength that further degrades transmission quality.